Range

Common broomweed, studied by the Range and Wildlife Management Department at Texas Tech University, periodically dominates native rangelands throughout the southern Great Plains. The severity of common broomweed infestations is dependent upon adequate winter soil moisture for seedling emergence and spring rains to extend plant survival through the bolting stage. The degree of broomweed infestations can also be related to past land use. Overgrazing or intensive mechanical brush control can reduce the competitive ability of perennial forage. If one or a combination of these conditions precede a moist fall and winter, the extent of common broomweed infestations is usually maximized. Once the seed base is built up, periodic infestion will continue despite good range management.

Management problems based on severe common broomweed infestations are numerous. Grass yield is usually reduced. Soil moisture used by broomweed is not available for grass production. Sunlight reaching the grass below broomweed canopies is reduced, thereby reducing grass production. Often grass production is reduced 50%, severely limiting the carrying capacity.

Toxicity to livestock has been reported for common broomweed, but not confirmed. Cattle avoid eating common broomweed, although sheep will graze it in the seedling and early bolting stages. Following canopy development, it is not usually grazed. Cattle avoid areas supporting dense thickets of common broomweed. Desirable forage beneath this canopy generally remains ungrazed, increasing grazing pressure on forage between broomweed thickets. The overall result is usually severe spot grazing.

Although common broomweed is an annual plant, the management implications from one broomweed year can persist for several years. Following senescence in the fall, the dead top growth is very resistant to breakdown. During the next year, dead broomweed canopies remain as a brittle barrier over the desirable forage. As livestock break through the canopy to graze desirable forage, the small side branches can break and lodge in the eye and nose, causing additional management problems.

Research conducted since 1976 in the Range and Wildlife Management Department at Texas Tech University has evaluated methods for controlling broomweed. We have effectively controlled broomweed with 2,4-D at 1/4 to 1 lb a.i./acre, dicamba and picloram plus 2,4,5-T at 1/8 to 1 lb a.i./acre. Common broomweed will germinate anytime during the winter or spring if soil moisture is adequate. As long as soil moisture is adequate, broomweed will over winter as seedlings or small rosettes. Infested pastures can be sprayed anytime from December through mid-May. However, if the spring is dry and the rainfall is below normal, most of the broomweed seedlings will die from drought. The most economical time to control common broomweed with herbicides is from mid-April to mid-May when the plants are actively growing. If broomweed is going to be a problem during any given year, it will usually be apparent by mid-April. After the "broom" begins to form, it is too late to spray. By the broom stage, herbicide effectiveness is minimal and damage to the livestock and the range is already prevalent.

Regardless of herbicide, date of the application is the key to effective control. Application during the seedling stage provides excellent control but it may not be profitable, since natural seedling mortality is often high and few plants survive to canopy development. Therefore, evaluation of the degree of common broomweed infestation should be made when the plants are bolting (early April) and chemical control should be initiated before canopy development. If common broomweed is to be controlled with herbicides, application should be made in late spring after the plants are tall enough to intercept chemical sprays. However, one should be aware that nonselective chemical application during late spring can kill many desirable perennial forbs, important in multiple-use management.

With the increased costs of herbicides, less expensive control alternatives warrant consideration. Dr. Henry Wright, fire ecologist at Texas Tech University, observed in 1969 that burning rangeland in March effectively controlled common broomweed. Thus, burning can provide a low-cost control of common broomweed when it can be justified in meeting management objectives. Successful broomweed suppression by fire is related to the amount and uniformity of grass cover as well as the weather conditions at the time of the burn. Prescribed burning provides excellent broomweed control if the site has enough grass to carry the fire. Broomweed generally survived in areas where the fire did not effectively burn, such as rocky slopes and swales with a high proportion of bare ground, or heavily grazed short grass areas. Minimum grass necessary to carry a fire is 1,000 lb/acre.

Equally important as site condition is the weather conditions at the time of the burn. Test sites burned before grass green-up in the spring resulted in 100% broomweed mortality when burning conditions were as follows: relative humidity 25 to 40%, winds 8 to 15 mph, and air temperatures 70 to 80° F. These conditions agree with those reported by Dr. Henry Wright. However, when environmental conditions are marginal, broomweed control is reduced. For example, some sites burned with lower air temperatures, higher relative humidities, and reduced wind speeds provided only 68% broomweed mortality. Although leaves of most broomweed seedlings are scorched similar to those in more successful burns, grass litter is not totally consumed. Apparently, temperatures on the upper surface of litter are high enough to scorch leaves but not high enough below the litter to kill the plants. When apical meristems are killed, apical dominance is eliminated, activating dormant axillary buds. New branches from these buds produce dense, multistemmed plants.

Burning, however, is not always feasible. On low producing or overgrazed sites herbaceous standing crop may be too small to carry a fire. Or, common broomweed infestation may occur during interim years in pastures already under a burning program. An additional burn at this time may be detrimental to the desirable forage. In these situations, herbicidal control is a logical alternative. If the target species is common broomweed, 2,4-D or dicamba provides a low-cost, effective control.

The decision to control common broomweed should be weighed against reduced calf crop, reduced weaning weights, reduced accessibility to available forage, and increased incidence of "pink eye" in cattle.

The major direct benefit the year of control is increased grass production which translates into increased carrying capacity. Although a rancher probably cannot increase his stocking rate following broomweed control, he would not have to reduce his stocking fate which could become necessary if he did not control broomweed. For example, in 1977 in the northern Rolling Plains, we obtained common broomweed and grass production of 650 and 950 lb/acre, respectively. Where the broom weed was controlled, its production was about 25 lb/acre, but grass production was over 1700 lb/acre. Assuming that 29,200 lbs of forage/year (80 lb/day of herbage production --- including intake, forage loss, and reserve) are required to carry an animal unit in the Rolling Plains of Texas, the carrying capacity on broomweed infested ranges would average 30 acres/AUY (animal unit year). Control of common broomweed and a concomitant increase of grass production (1.8-fold) would reduce the number of acres to support an animal unit to 17 acres/AUY. If common broomweed is not controlled during years of high infestations, ranchers should probably reduce their stocking rates by one-third to avoid reduced animal performance and damage to the range resource.

Treatment³	Application rate (lb a.i./acre)⁴	Application time
2,4-D	1/4 to 1	Mid-April to Mid-May
Dicamba	1/8 to 1	Mid-April to Mid-May
Picloram + 2,4,5-T	1/8 to 1	Mid-April to Mid-May
Burn		March

¹ Contribution No. T-9-337 of the College of Agricultural Sciences, Texas Tech University.

² Dr. Gordon's current address is Lilly Research Laboratories, 307 E. McCarty St., Indianapolis, Indiana 46206.